Molded articles of a fiber reinforced resin whose strength is increased by containing reinforcing fibers are used for various purposes. To obtain the molded articles by injection molding, a thermoplastic resin is melted by rotating a screw in a cylinder that is a plasticizing device, kneaded with fibers, and then injected into a mold of an injection molding apparatus.
In order to obtain the effect of improving strength by the reinforcing fibers, the reinforcing fibers are desired to be dispersed uniformly in the resin. Although a strict kneading condition may be employed for achieving the uniform dispersion, a shear force on a kneaded material is inversely increased, and the reinforcing fibers are thereby cut off. Thus, a fiber length after molding becomes significantly shorter than an original fiber length, and the obtained fiber reinforced resin molded article may not be able to satisfy a desired property (Patent Literature 1). Therefore, it is necessary to select an injection molding condition under which an appropriate shear force is obtained at the time of kneading.
Also, in order to prevent the fibers from being cut off in a plasticizing process, there is proposed a plasticizing device which includes a cylinder separately provided with a feed port for feeding a thermoplastic resin raw material to the cylinder, a feed port for feeding a fiber material, and a vent port for removing a gas content contained in the resin, and a screw (Patent Literature 2).
The fibers of the fiber reinforced resin are cut off in the plasticizing process mainly in a front half of the plasticizing device, that is, a feed section and a compression section of a resin pellet. Thus, in the plasticizing device, the feed port for a fiber material is provided downstream of the compression section where the thermoplastic resin is sufficiently melted.